It is hoped that this investigation will lead to a better understanding of the regulation of protein synthesis and degradation under normal conditions. The study of how it is altered in an attempt by the organism to meet the nutritional crisis brought about the lack of protein or calories is important and imperative. The rates of protein synthesis will be measured by the constant infusion of (14C) tyrosine. The fractional rates of protein breakdown will be estimated in muscle from the decay in (1 14C) glutamate radioactivity and in liver from the decay in amino acids labeled by (14C) carbonate. Both tracers will also be used as the C14 label in the development of double labeling procedures to measure absolute degradative rates. The mechanisms of regulation of protein synthesis and degradation will be sought from the study of protein turnover of total tissue proteins and, in particular, of single, isolated proteins wherever possible in rats during nutritional deprivation, in two strains of rats with significantly different growth rates, in rats of different age, and in animals of different size, viz, rats and swine. The role of growth hormone and dietary protein in this regulation and the role of the lysosomes in protein breakdown will also be explored.